1. Field of the Invention
The present invention relates, in general, to a permeameter for in-situ measurement of saturated hydraulic conductivity of streambed sediments. The head difference between head within the chamber and head at the inlet of the pipe acts as the driving force to flow water from the chamber to the pipe. This head difference can be induced by adjusting the depth of the inlet in the pipe below stream stage. Water in a saturated zone around the chamber flows upwards through the sediments inside the chamber without electric power.
Upward flowing of water within the chamber causes a hydraulic head difference between the upper side and lower side of the chamber. In situ saturated hydraulic conductivity of the streambed sediment can be determined by measuring this hydraulic head difference in the chamber and an amount of water passing through the inside of the chamber.
2. Description of the Related Art
Generally, it is recognized that surface water and groundwater are resources that are interconnected and interact with each other so that physical and chemical changes in one affect physical and chemical composition/state of the other. Surface water-ground water interaction is an essential factor to be evaluated for purposes such as water budget analysis, understanding of passing routes of nutrients, contaminants, and investigation of groundwater flowing system, etc.
The interaction between the surface water and groundwater means discharge of groundwater from aquifer toward surface water, or recharge of the surface water into the aquifer. In order to evaluate the interaction between surface water and groundwater quantitatively, measurements for the hydraulic conductivity of streambed sediment, a hydraulic gradient between surface water and groundwater, and direct or indirect measurement of an amount of exchange between surface water and groundwater are needed. The hydraulic conductivity, i.e. a target object to be measured, is a value for a physical property that, like electric conductivity or thermal conductivity, is a measure of how well an aquifer substance allows water to pass therethrough. The hydraulic conductivity has a functional relationship with a porous medium and a fluid passing through the porous medium. The hydraulic conductivity of the porous medium is obtained using Darcy's law. Darcy's law is a constitutive equation that is formulated based on experimental researches on the flow of water through beds of sand, wherein groundwater flows from upper side toward lower side of a hydraulic head through the porous medium, and the flow rate of the groundwater is proportional to the hydraulic conductivity and hydraulic gradient. Hydraulic head is a specific measurement of liquid pressure above a geodetic datum. Hydraulic head of groundwater is expressed by the sum of an elevation head, a pressure head, and a velocity head. However, the velocity head may be ignored because generally the flow of groundwater is very slow. In an experimental apparatus (FIG. 1) for explaining Darcy's law, it can be seen that a flowing direction of groundwater is determined by a difference (ΔH) in hydraulic heads of left and right water tanks, which are connected to a sand tank, irrespective of a position of the sand tank. In FIG. 1, when a datum level where z=0 is set, a hydraulic head H1 of an A-sided water tank that amounts to the sum of an elevation head Z1 and a pressure head P1 is higher than a hydraulic head of a B-sided water tank (H2=Z2+P2), so that it is shown that the flowing direction of groundwater is oriented from the A-sided tank of higher head toward the B-sided tank of lower head. Here, an amount Q of water passing through the sand tank is proportional to a difference in hydraulic heads (ΔH=H2−H1), a cross-section area A of the sand tank, and the time t elapsed, and is inversely proportional to a length L of the sand tank (Equation 1)
                    Q        ≈                              -                                          (                                                      H                    2                                    -                                      H                    1                                                  )                            L                                ⁢          At                                    Equation        ⁢                                  ⁢        1            
If a proportional factor K is introduced, the Equation 1 can be expressed as following Equation 2.
                    Q        =                              -            K                    ⁢                                    (                                                H                  2                                -                                  H                  1                                            )                        L                    ⁢          At                                    Equation        ⁢                                  ⁢        2            
The constant of proportionality K is called the hydraulic conductivity that depends both on characteristics of a fluid passing through a porous medium and porous medium itself.
Here, (H2−H1)/L is a hydraulic gradient.
The present invention relates to an apparatus for measuring hydraulic conductivity of sediment in a saturated zone of a water-sediment interface by measuring a quantity Q of groundwater passing through a chamber, which is inserted into sediment under a water-sediment interface, and a hydraulic gradient between two points within the chamber. Here, the saturated zone means a section where pores of porous medium are filled with groundwater, whereas the unsaturated zone means an air-permeable section that is ventilated above the surface of groundwater.
Generally, the saturated hydraulic conductivity of sediment under a water-sediment interface may be measured by a physical method and an estimating method using e.g. a tracer, modeling, or the like. The physical method is performed using a standpipe opening at both sides, which is inserted into sediment under the water-sediment interface, and includes a method using a constant head or falling head, a coupled method using a seepage flux and a hydraulic gradient between surface water and groundwater, a slug test, a grain size analysis and the like. It is evaluated that the physical method is a more economical than the estimating method like a tracer test or modeling. However, it has been reported that methods using the standpipe and seepage meter have problems of reduced reliability in terms of measuring results, the slug test has problems in that only horizontal hydraulic conductivity can be obtained, and the grain size analysis method has problems of using empirical formulae (Landon et al., 2001). Particularly, in the case of the standpipe method that directly measures the hydraulic conductivity of the streambed sediment in the field, water is introduced into the top of the standpipe, which is inserted into the streambed sediment that is a target object to be measured for hydraulic conductivity, so as to cause water to flow downwards through sediments within the pipe, thereby measuring the hydraulic conductivity in a manner of using a constant head or a falling head.
However, an apparatus and method of in-situ measuring of saturated hydraulic conductivity which causes water to flow upwards through sediment within chamber or pipe with or without power are not yet provided.